<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>

        //原始数据类型
        alert(typeof 3);    //number
        alert(typeof 3.14);    //number

        alert(typeof"A");       //string
        alert(typeof"HELLO");   //string
        alert(typeof"Hello JS");//string
               
        alert(typeof true);     //boolean
        alert(typeof"false");   //boolean
               
        alert(typeof null);     //object        
        
        let a;
        alert(typeof a);        //undefined
    </script>
</body>
</html>